Generally, a lubricant is used at a sliding portion and a rotation portion of most machines represented by cars and machines for industrial use. The lubricant is classified into a liquid lubricant and a solid lubricant. Grease obtained by thickening the lubricant to have a shape retention property and a solid lubricant which holds the liquid lubricant and is capable of preventing the liquid lubricant from scattering and dropping are also known.
In recent years, technical improvement for allowing cars to have a high performance, be compact, and be lightweight has progressed. Thus there is an increase of demands for allowing a universal joint such as a constant velocity joint used to transmit driving forces of industrial machines and component parts of cars to be compact, have a high performance, and have a long life.
Because similarly to other machines, demands for allowing the universal joint to be compact and lightweight have increased, a load applied to the universal joint has become higher. In this case, it is difficult to allow the universal joint to have a sufficiently long life by lubrication with grease. Therefore it is preferable to allow the universal joint to have a long life by a simple maintenance of incorporating the solid lubricant therein.
For example, solid lubricants containing a lubricating oil or grease mixed with an ultra-high-molecular-weight polyolefin or urethane resin and a hardener therefor to hold a liquid lubricating component between molecules of the resin so that the solid lubricants have properties of exuding the lubricating component gradually (see patent documents 1 through 3) are known.
In addition, a polyurethane elastomer having self-lubricating property obtained by a reaction between polyol and diisocyanate, both of which are materials of polyurethane, in a lubricating component is known (see patent documents 4).
When these solid lubricants are solidified by enclosing them in a bearing, the lubricating oil gradually exudes therefrom. By using the solid lubricants, it is unnecessary to perform maintenance for replenishing the lubricating oil and frequently allow the bearing to have a long life in a strict environment where much water is present and environment where a strong inertial force works.
But when the above-described solid lubricant is used at a portion such as the driving portion of the constant velocity joint where an external force such as compression, flexure, and the like is repeatedly applied at a high frequency, a very large force is necessary to deform the solid lubricant in conformity to the compression, flexure, and the like or a very high stress is applied to the solid lubricant. Therefore a portion holding the solid lubricant requires a high mechanical strength. But the strength of the solid lubricant and the filling rate thereof are usually contrary to each other. Therefore it is difficult to hold the solid lubricant at a high filling rate. Thus there is a possibility of preventing the bearing from having a long life.
Therefore there is a demand for the development of a solid lubricant which can be easily used at the portion to which the external force such as compression, flexure, and the like is repeatedly applied at a high frequency. As an example of the solid lubricant for complying with such a demand, the solid lubricant consisting of an oil-containing foam in which the lubricating oil is retained in pores of the foamed flexible resin that is used by filling it in a bearing and a constant velocity joint is known (see patent documents 5).
But the solid lubricants disclosed in the patent documents 1 through 4 have a high lubricating oil-retaining performance, but lack a flexible deformability. The solid lubricant disclosed in the patent document 5 has a flexible deformability adaptable to the external force and is capable of following deformation caused by compression and flexure. But the solid lubricant has a low lubricating oil-retaining performance. Thus when the solid lubricant is used for a bearing operating in a condition of a high speed, there is a possibility that the lubricating oil is rapidly discharged from the solid lubricant and exhausted.
The above-described solid lubricant can be used in a short period of time and an airtight space. When the solid lubricant is used in a portion required to be lubricated for a long time and an open space, the lubricating oil scatters from the open portion of the solid lubricant. Thus there is a case where the supply of the lubricating oil is short. When the oil-retaining performance is low, an excess of lubricating oil is repeatedly discharged from pores and absorbed therein, thus always flowing in the space.
When an excess of the lubricating oil which has exuded from the solid lubricant contacts an exterior material such as rubber, in some cases, the exterior material may be chemically corroded or deteriorated by the lubricating oil and additives thereof.
In the method of producing the solid lubricant, many production steps are required to securely impregnate the solid lubricant with the lubricating oil and grease. Thus it is difficult to comply with the demand for the development of the solid lubricant that can be produced at a low cost.    Patent document 1: Japanese Patent Application Laid-Open No. 6-41569    Patent document 2: Japanese Patent Application Laid-Open No. 6-172770    Patent document 3: Japanese Patent Application Laid-Open No. 2000-319681    Patent document 4: Japanese Patent Application Laid-Open No. 11-286601    Patent document 5: Japanese Patent Application Laid-Open No. 9-42297